1. Field of the Invention
The present invention relates to a method for controlling the thickness of a crystal film in which the thickness of a thin film which is being grown in a vacuum system is measured in terms of mono-layer unit so that the thin crystal film is grown until the desired number of mono-layers is counted layer by layer and a method for growing a standard mixed crystal having a correct composition ratio by correctly determining the composition ratio of a mixed crystal.
2. Description of the Prior Art
Of the various methods for measuring the thickness of a crystal which is being grown in a vacuum system, there is a method using a film thickness measuring instrument incorporating a quartz oscillator. In this method, the thickness of a film or layer can be measured with a higher degree of accuracy, but it is needed that the growth rate at not only the position of the quartz oscillator but also at the position of a substrate upon which a film or layer is grown must be accurately calibrated. However, this method cannot be applied to the measurement of the thickness of a film or layer of a material such as GaAs whose sticking coefficient is less than 1. In this case, the thickness of the grown layer is measured with an interference microscope or the like only after the layer of such material as described above is grown and is taken out of the vacuum system. However, even when this measuring method is used, it is still difficult to measure the thickness of a layer or film with an accuracy less than 100 .ANG..
As described above, it is difficult to correctly measure and control the thickness of a crystal film while the film is growing.
Meanwhile, J. J. Harris et al. of Philips Research Laboratory in England found out that when the diffraction patterns of a GaAs film grown over a GaAs substrate by a molecular beam epitaxy process under ultra-high vacuum conditions, in which the electron beam from a reflection high energy electron diffraction (RHEED) system is directed in the [110 ] azimuth (which includes [110], [110] and [110 ] azimuth) of the (001) GaAs substrate, are observed, the intensity of the diffraction pattern is periodically varied and one period of the intensity oscillation corresponds to the growth of one mono-layer of GaAs on the (001) plane. In this case, however, the one mono-layer consists of a complete monoatomic Ga layer and a complete monoatomic As layer and the thickness of the mono-layer is equal to one half of a lattice constant. (See Surface Science, Vol. 103, L90-L96, 1981.) It follows, therefore, that the number of mono-layers grown can be exactly measured by counting the oscillations of the intensity of the diffraction pattern.
The above-described experimental results can be appreciated as follows. FIG. 1A shows the periodic oscillations of the intensity at the specular point of a typical diffraction pattern and FIG. 1B shows a model of the surface conditions of the grown films corresponding to points .circle.1 - .circle.9 the oscillation shown in FIG. 1A. In FIG. 1B, .theta. represents a fractional layer coverage. .circle.1 shows a mirror surface condition (.theta.=0) and .circle.2 shows .theta.=0.25. As the coverage .theta. is increased, the mirror-surface-reflection intensity of the diffraction pattern is decreased and when .theta.=0.5 as shown at .circle.3 , the intensity becomes a minimum value.
When 0.5&lt;.theta.&lt;1.0 like .theta.=0.75 as shown the mirror-surface-reflection intensity is increased with the increased with the increase of .theta. and becomes a maximum value when .theta.=1 as shown at .circle.5 . This tendency is repeated at the same period from .circle.5 to .circle.9 so that whenever one mono-layer of semiconductor thin film is grown, the intensity of the diffraction pattern oscillates once.
However, it is reported that such oscillations are damped after several tens of oscillations (J. H. Neave et al., applied Physics A31 pp. 1-8, 1983). Therefore, it is only possible to measure the thickness of a grown layer up to about 100.ANG. in terms of the number of oscillations, so that the above-described measurement method is not satisfactory in practice in case of crystal growth for a device requiring a thicker film.
With the above in view, the thickness of a layer or film being grown is controlled by the time during which the shutter of a crystal growth apparatus is kept opened, in the case of crystal growth under vacuum conditions (which is simply referred to as "crystal growth" hereinafter in this specification), in either case where a thick or thin film is required. Furthermore, the mixed crystal composition ratio (to be referred as "the composition ratio" hereinafter in this specification) of a mixed crystal is determined by an analysis of X-ray diffraction or by other composition analizer after the crystal has been grown. This will be described in more detail with reference to the drawings.
FIG. 2A is a schematic view of a crystal growth apparatus. Reference numeral 1 denotes a crystal substrate (to be referred to as "the substrate" hereinafter in this specification); 2, a heater for heating the substrate 1; and 3, materials (to be referred to as "the source" hereinafter in this specification) required for the crystal growth. In the case of GaAs, the materials are Ga and As or dopants such as Si and Be. When the sources in the form of ions is needed, an ion gun 4 must be provided. Furthermore, when the source has a high melting point, an electron gun or sputtering apparatus must be used. Moreover, when the ions are in the form of a gas, a gas reservoir 6 and a flow-rate controller 7 are required. Each source is provided with a cell shutter (to be referred to as "shutter" hereinafter in this specification) in order to discharge or interrupt the source vapor (to be referred to as "flux" hereinafter in this specification).
Reference numeral 9 denotes a main shutter for permitting or interrupting the arrival of all source fluxes to the substrate 1. Each shutter 8 is operatively coupled to a shutter actuating device 10. Reference numeral 11 denotes a vacuum gauge or flux monitor for detecting the amount of each source flux.
Reference numeral 12 denotes a gun of a reflection high energy electron diffraction system (to be referred to as "RHEED" hereinafter in this specification) and the electrons 13 emitted from the gun 12 pass over the surface of the substrate 1 and impinges against a fluorescent screen 14 (to be referred to as "screen" hereinafter in this specification) of the RHEED. In this case, the electrons 13 are diffracted by the substrate 1 so that a diffraction pattern is displayed on the screen 14. This pattern is referred to as "RHEED pattern". An example of the pattern photograph is shown in FIG. 2B. The above-described devices and component parts are all enclosed in a vacuum chamber 15.
The process for growing a crystal on the substrate 1 will be described. First, the substrate 1 is heated at a suitable temperature by the heater 2 and then the shutters 8 of the required sources 3 and the main shutter 9 are opened. On the other hand, when the crystal growth is interrupted, the shutters 8 and the main shutter 9 are closed. In order to grow a crystal of GaAs or AlGaAs, the As flux is normally opened while the shutter for Ga or Al is opened or closed to start or terminate the crystal growth. In the case of a mixed crystal, both the shutters for Ga and for Al are simultaneously opened or closed to start or interrupt the crystal growth. Hereinafter, explanation will be made of GaAs, AlAs and AlGaAs as a mixed crystal.
So far, in order to measure the thickness of a grown crystal, Ga, Al and As fluxes are previously measured by the flux monitor 11 and then the thickness (5000.ANG.-2 .mu.m) of a crystal which has been grown for a long time (about one hour) with the fluxes is measured by an optical interference microscope or thin-film thickness gauge so as to prepare the calibration table of the growth rate showing the relationship between the growth rate of a thin film and a flux. Thereafter, the thickness of a grown film is estimated from the growth rate obtained from the calibration table by using only the flux value and the growth time. In order to measure the thickness of a few .ANG. to hundreds .ANG. which is substantially equal to the thickness of one mono-layer to ten mono-layers which can be grown within a short period of time, the thickness is also estimated from the flux value and the growth time based on the calibration table of the growth rate obtained in the manner described above by the long time growth. However, according to this method, the variation in growth rate due to the variations of flux in time immediately after the shutter is opened is not taken into consideration, and therefor it is difficult to control the thickness of a film comparable to a mono-layer.
FIG. 3 shows the variation of the Ga flux with time measured immediately after the shutter is opened.
The Ga flux varies for a few minutes after the shutter has been opened, so that during this period of time the growth rate is not constant. As a result, the prior art method for measuring the thickness of a grown film based on the calibration table of the growth rate cannot be effectively used until the time that the growth rate becomes constant. Especially, in the case of the fabrication of a device such as a superlattice, the shutter must be opened and closed within a few seconds or a few minutes, i.e., before the growth rate becomes constant, so that the thickness of the grown film cannot be measured.
FIG. 4 schematically shows an example in which two kinds of crystals A and B are alternately grown as mono-layers in the manner described above ad it is seen that the interface between the mono-layers is not flat and fluctuated.
In the case of a mixed crystal growth such as Al.sub.x Ga.sub.l-x As, the mixed crystal composition ratio x must be determined. In order to determined the value x, the following method has been employed. First, prior to the crystal growth, the Ga, Al and As fluxes are measured by the flux monitor 11 shown in FIG. 2 and the growth rate of GaAs and the growth rate of AlAs are measured, whereby the calibration table is prepared. Thereafter, the simultaneous growth rate and the composition ratio x are obtained from the growth rates of GaAs and AlAs and after the mixed crystal growth, the lattice constant is measured by X-ray diffraction, while the composition is analyzed, so that the value which can explain any data without any contradiction is determined as the composition ratio x. The above-described prior art method for determining the composition ratio is also based on the growth rate of a crystal which has been grown for a long time. Therefore, while the flux has no constant value, the composition ratio cannot be determined.
As described above, when the growth rate which is required in the control of the thickness of a grown layer and in the determination of the composition ratio is determined, considerable difficulties are encountered in practice when the thickness of a film or layer, which is most important, is determined by the prior art based upon a long-time growth. As a result, thickness of a grown film cannot be obtained with a desired degree of accuracy. Thus, the significant figure has one or two digits. The accuracy of the thickness of a grown film and the composition ratio measured by the above-described prior art method are two digits at the most. Furthermore, in the case of the measurement of the thickness and composition of a crystal which is grown for a short period of time, the accuracy is further low. If the composition ratio can be obtained at a higher degree of accuracy in principle, a standard crystal with a predetermined composition can be obtained.
According to the prior art method for controlling the thickness of a grown film, the thickness of the grown film cannot be determined and the heterojunction interface is indefinite. Furthermore, since the composition ratio of a mixed crystal is not determined, it is difficult to fabricate a device as designed.
As a result, the laser wavelength of a quantum well laser or a multi-quantum well laser is deviated or divided into a few wavelengths so that the laser wavelength itself cannot be realized as designed. In the case of a light emitting diode, the wavelength of an emitted light is deviated from a designed value and the efficiency is degraded. In the case of HEMT (high electron mobility transistor) and a device in which the conduction of two dimensional electrons is utilized, the interface is disturbed and the potential step difference is deviated from a designed value, so that the scattering at the interface occurs and the concentration of two dimensional electrons is deviated from a designed value. As a result, high speed operation cannot be expected.
Furthermore, in the cases of vertical superlattice devices such as one barrier diodes, two-barrier diodes, CHIRP (Coherent Hetero Interfaces for Reflection and Penetration) superlattice (See U.S. Ser. No. 578,574) or the like and the superlattice gate field-effect transistors, their characteristics vary considerably one from another, because of the indefinite interface and the deviations from the designed values.